The front side metallization of semiconductor devices has the task of specifying a reliable low-resistance contact with the integrated circuit elements of the semiconductor chips. The reliability and usability of present-day front side metallizations are limited, however.
Thermal loading on packaged semiconductor chips may cause cracking at the boundaries between the different materials of the device. Said cracking is caused by the difference between the coefficients of thermal expansion of the materials used and may lead to the failure of the device. The cracking may occur at the boundary between the semiconductor chip and the plastic housing composition and also between the metallic circuit carrier and the plastic housing composition.
It is generally known that cracks can occur between the metallization and the dielectric passivation on the semiconductor chip. Cracks can also form in the passivation layer and grow and propagate from the passivation layer into the semiconductor chip. This problem is described for example in “Thin film cracking and ratcheting caused by temperature cycling”, M. Huang and Z. Suo, J. Mater. Res. Vol. 15, No. 6, June 2000.
It is also known from this document that cracking at the boundaries between the metallization and the passivation layer on a semiconductor chip constitutes an increasing problem in the case of continuous temperature cycling.
In addition to improved temperature stability there is also the need for a metallization which can be connected more simply and more reliably to the contact elements, e.g. bonding wires, of the device and also to the contact areas of an external printed circuit board.
U.S. Pat. No. 4,132,813 discloses applying an Ni layer on an Al—Ni front side metallization. An Ni layer has the advantage that it can be wetted by soft solder. The temperature stability of this metallization is inadequate, however, as mentioned above, since the thick Al metallization layer leads to cracking in the passivation layer.